The present invention relates to exothermic processes such as the water gas shift reaction and, more particularly, to an energy efficient method for conducting a multi-stage water gas shift reaction which includes simultaneous generation of process steam and electrical power.
Various type industrial process plants, e.g., petrochemical plants, refineries, and the like, utilize process steam and electrical power for plant operation and generate substantial quantities of process heat. Traditionally, the process heat sources are employed to produce saturated steam at pressure levels suitable for process use and any surplus steam is expanded through low-pressure steam turbines to generate a portion of the plant electrical demand. Although such a method of integrating the plant utility functions surely improves, to a point, the overall energy utilization of the plant, in this age of very expensive and ever-increasing energy costs, the traditional approach to utilities integration as a means of energy economy has been found seriously wanting. Indeed, no longer can energy economy be viewed simply as a matter of most efficiently utilizing whatever energy a process plant may produce. Rather, process parameters must be re-examined and processes reconfigured with an eye toward developing new methods for operating the processes in order to increase the effectiveness of process energy utilization above what was perhaps heretofore thought possible. In this way, by integrating process parameter selection with process energy utilization efficiency, the overall efficiency of industrial processes can be significantly improved.
A fundamental step in improving the overall energy efficiency of industrial processes, for example direct coal liquefaction, coal gasification, ammonia synthesis, methanation, catalytic oxidation such as SO.sub.2 +1/2O.sub.2 .fwdarw.SO.sub.3, and the like, is to focus on improving the overall energy efficiency of exothermic sub-processes or unit process thereof which have wide applicability to many industrial processes. One example of such a unit process, which traditionally has been operated in an energy inefficient fashion, is the well-known water-gas shift reaction process: EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2
This process reacts carbon monoxide gas (CO) with steam to produce a mixture of hydrogen and carbon dioxide gases. The reaction is exothermic and typically employs a catalyst, such as iron, iron compounds (e.g., oxides), chromium, chromium compounds (e.g., oxides), mixtures thereof, and other catalyst materials well known to the art. The rate of conversion of the reaction is accelerated at higher temperatures while the extent of conversion is enhanced by lower reaction temperatures. When the carbon dioxide is separated from the product gas mixture, such as by use of carbonate-forming systems, ethanolamine absorbents, or other known means for removing carbon dioxide, a gas stream having a higher concentration of hydrogen is obtained.
The water gas shift reaction is typically carried out using either a single stage shift reactor or a multi-stage, generally two stage, shift reactor to attain the desired degree and rate of conversion of carbon monoxide and steam to hydrogen. In carrying out the water gas shift reaction in a single stage, steam is introduced into the shift reactor together with the carbon monoxide-containing gaseous stream, which typically also includes hydrogen and carbon dioxide, in the presence of an appropriately selected catalyst. The single stage water gas shift reaction is exothermic and favored by inlet temperatures generally in the range 450.degree. to 900.degree. F. The gaseous effluent leaving the shift reactor, consisting essentially of carbon dioxide and hydrogen gas, is typically subjected to a carbon dioxide removal process to increase the hydrogen concentration in the product stream. In a conventional two-stage water gas shift conversion process, two catalytic shift reactors, operating at high and low temperatures respectively, are used to attain the desired degree of conversion to hydrogen. A carbon monoxide-containing feed stream, typically including hydrogen and carbon dioxide, is first fed to the high-temperature shift reactor. The water vapor concentration required for the shift reaction is frequently added to the feed gas stream upstream of the high-temperature shift reactor by quenching the feed gas with water to cool the normally very hot feed gas stream to a lower temperature. In some instances water in the form of steam may be added directly to the shift reactor. The partially shifted gas stream exiting the high-temperature shift reactor, at a temperature higher than that of the entering feed gas stream as a result of the exothermic nature of the shift reaction, is cooled to a temperature level appropriate for the low-temperature shift reaction stage and introduced into the low temperature shift reactor. The fully shifted product gas stream exiting the low-temperature shift reactor is cooled to a temperature appropriate for the separation and removal of carbon dioxide and carbon dioxide is removed therefrom to produce a hydrogen-rich product gas stream.
Whether the water gas shift reaction is carried out in one or multiple stages of shift reaction, the conventional approach is to select process parameters and configurations which optimize yield which at the same time utilizing whatever process steam and/or process energy may become available as a result of operating at the selected process parameters and supplementing the available process steam and/or energy with on-site produced steam and electrical generation, for example by combustion of fuel. It has now been found, as illustrated by the invention described and claimed in the present application, that selecting processes parameters and configurations to optimize process yield does not always optimize overall process economics. Indeed, the very high cost of energy frequently dictates the selection of process parameters and configurations which maximize and/or optimize process energy production, recovery and economic utilization to achieve a significant improvement in overall process economics. This is particularly so in connection with the conduct of the water gas shift reaction process.
It is therefore an object of the present invention to provide a process for the conduct of a multi-stage water gas shift reaction which maximizes and/or optimizes process energy production, recovery and utilization efficiency to achieve significantly improved overall process economics as compared with heretofore conventional water gas shift reaction processes.